Definition

Lymphocytosis (lim-foe-sie-TOE-sis), or a high lymphocyte count, is an increase in white blood cells called lymphocytes. Lymphocytes help fight off diseases, so it's normal to see a temporary increase after an infection.

A count significantly higher than 3,000 lymphocytes in a microliter of blood is generally considered to be lymphocytosis in adults. In children, the threshold for lymphocytosis varies with age. It can be as high as 9,000 lymphocytes per microliter. The exact thresholds for lymphocytosis can vary slightly from one lab to another.